1. Field of the Invention
The present invention relates to an apparatus for measuring current density of a fuel cell. The fuel cell includes an electrolyte electrode assembly, and separators for sandwiching the electrolyte electrode assembly. Each of the electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of a fuel cell (unit cell) for generating electricity. Typically, a predetermined number of membrane electrode assemblies and separators are connected together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane which is appropriately humidified, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field (reactant gas flow field) is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field (reactant gas flow field) is formed on a surface of the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between the adjacent separators for supplying a coolant for cooling the fuel cell as necessary.
During power generation, water produced in the chemical reaction is likely to be trapped on the outlet side of the reactant gas flow fields, and the electrolyte membrane may be humidified excessively. If water flooding occurs, reactant gases may not be supplied to the electrode surfaces sufficiently.
In an attempt to address the problem, current density in the electrode surface is monitored for detecting a local current density drop due to flooding. For example, Japanese Laid-Open Patent Publication No. 8-222260, entitled, “ABNORMALITY MONITORING METHOD OF FUEL CELL AND DEVICE THEREOF” discloses an abnormality monitoring method for detecting an abnormal condition of a fuel cell. The abnormal condition is detected by measuring distribution of reducing gas concentration of the fuel gas in the anode surface, or distribution of oxidizing gas concentration of the air in the cathode surface.
Further, Japanese Laid-Open Patent Publication No. 9-223512, entitled, “ABNORMALITY MONITORING METHOD OF FUEL CELL AND DEVICE THEREOF” discloses an abnormality monitoring method for detecting an abnormal condition of a fuel cell. A load current of the fuel cell is changed gradually, and temperature at an arbitrary position in the surface of the base material of the anode, the base material of the cathode, the reactant gas flow field (grooves), or the electrolyte layer is measured. During power generation of the fuel cell, amount of heat energy generated at the arbitrary position is determined based on the relationship between the gradual change of the temperature and the gradual change of the load current to measure density of electric current in the electrode surface. In this manner, the abnormal condition of the fuel cell is detected.
According to the disclosure of Japanese Laid-Open Patent Publication No. 8-222260, a stainless thin tube is connected to a vacuum chamber through a leak control valve. A measuring element of a quadrupole mass spectrometer is provided in the vacuum chamber. The stainless thin tube is inserted in one of fuel gas flow grooves for measuring hydrogen concentration in the fuel gas flow groove. Then, hydrogen concentration in the next fuel gas flow groove is measured by inserting the stainless thin tube in the next fuel gas flow groove. The measuring process is performed repeatedly for measuring hydrogen concentration in each of the fuel gas flow grooves.
In the technique of Japanese Laid-Open Patent Publication No. 8-222260, however, if there are many fuel gas grooves, since it is necessary to insert the stainless thin tube in each of the fuel gas grooves for measuring hydrogen concentration, the abnormality detecting operation is laborious, and time consuming.
In the technique of Japanese Laid-Open Patent Publication No. 9-223512, amount of heat energy generated at the arbitrary position is determined based on the relationship between the gradual change of the temperature and the gradual change of the load current to measure density of electric current in the electrode surface. Thus, it takes considerable time to measure density of electric current in the electrode surface during power generation. The abnormality detecting operation can not be performed efficiently.